How can I make a hexagonal grill?
Here's a way to do this using modifiers:


Add a circle (&#8679; ShiftA> Mesh > Circle) and set the number of Vertices to 6 in the redo panel to make a hexagon:



Take note of the X dimension of the object. With the default radius of 1 it should be 1.732:


Add an Array modifier with a relative offset of .5 and .75 to create a tile-able section with two offset hexagons:


Add an array modifier to tile the object along the X axis. 

Note the X dimension of the object now with the modifier. It should be equal to original_x+(original_x/2) (2.598 with the default radius): 



Set the relative X offset of the array modifier to original_x / modified_x), or .666...:


Add yet another array modifier to tile the object along the Y axis, with a Y offset of .857, or radius*3 / (original_y+original_y/2).
Increase the Count of the last two array modifiers as desired:


Now you have a wireframe, but it needs thickness. Switch to edit mode (&#8633; Tab), select all (A) and extrude (E). Cancel the transform part of the extrude (Esc) and scale down (S):


Enable Merge on all the array modifiers.
Add a solidify modifier (and optionally a bevel modifier):






In case you might want something more rounded, you can adjust the angle limit on the bevel modifier and toss a subsurf modifier on the end: